I'm using Firefox 63.0.3 on Fedora 29 Linux. When I download an image in .FITS format (standard in astronomy), I get the "What should Firefox do with this file?" dialog box, and select "Open with". The suggested application is Gimp, which is useless for my purposes; I want the astronomical image viewer ds9, a descendant of SAOimage. The dropdown menu does not offer ds9 as a choice, so I select "other", which opens a "Select Application" dialog box. This also does not list ds9, nor is it in the "View all applications" list, so I go to "Find new applications" which offers me the same selections in fancier format, but also has a search box. I put the path to ds9 (/usr/local/bin/ds9) in the dialog box, but the choice is not offered -- I get "No applications found" anywhere along the path.

This used to work. What seems to have happened is that a design decision was taken to create a special menu of Firefox-approved 'applications' and limit the user's choices to these alone. Is there any workaround, or am I stuck with downloading files I don't want to save and laboriously opening them by hand in a separate application? This is driving me crazy, or more accurately, even crazier.